Developers used for electrophotographic image forming apparatuses such as copiers and printers include single-component developers incorporating only a toner and two-component developers incorporating a magnetic powder referred to as a carrier and a toner. Image formation using a two-component developer is an advantageous system to carry out high-speed print production, since, due to the presence of a carrier, rapid toner charging can be carried out.
A carrier used in a two-component developer charges a toner as described above, and needs to have stable charge providing performance over a long term even when repeatedly used for tone charging. Such a carrier is constituted of a magnetic particle referred to as a core material (core), and its form includes one in which a core material is used as a carrier as is, as well as another one referred to as a resin coated carrier structured by coating the core material surface with a thermoplastic resin.
Such a resin coated carrier has a structure in which the core particle surface is coated with a resin and thereby has excellent durability and stable friction chargeability. Further, a technology, in which a resin is coated with no organic solvent, has been established (for example, refer to Patent Document 1). The production method of Patent Document 1 realizes uniform and tight bonding of a resin particle to the core particle surface using a stirring action via rotation of a rotary vane and thereby can provide a resin coated carrier having a uniform, adequately thick resin layer and exhibiting excellent durability and stable friction charge providing performance.
Incidentally, in an electrophotographic image forming apparatus, with an increase in print production speed and size reduction of the apparatus, the stirring action in the developing device has become strong and such vigorous stirring has increased the impact applied to the developer. For example, during stirring, a violent collision between a toner and a carrier has occurred and thereby the collision causes toner particle breakage and external additive burial, whereby the toner has been unable to be charged at a predetermined level. As a result, there have been produced problems such as occurrence of image failure such as fog and transfer failure due to poor charging of the toner, a decrease in charging performance due to adhesion of toner-fractured materials to the carrier particle surface, and occurrence of contamination inside the machine caused by scattered materials. Further, with regard to a resin coated carrier particle having been subjected to a strong impact, a resin layer formed on the surface thereof is peeled of and then the resistance of the carrier is markedly decreased, whereby an adverse effect on charging for the toner has been produced.
For these problems, a technology, in which the weight of a carrier particle is reduced and thereby the impact applied to a developer during stirring is allowed to decrease, has been investigated. For example, a magnetic powder dispersed carrier in which fine magnetic power is dispersed in a resin has been proposed (for example, refer to Patent Document 2). However, in such a magnetic powder dispersed carrier, since a binder resin covers the magnetic powder, carrier resistance increases and thereby the toner is inadequately charged, resulting in the difficulty of formation of a toner image having sufficient image density. Further, in high-speed print production, due to an impact caused by stirring, the resin is peeled off and then the magnetic powder is removed, whereby problems have been produced in stable toner image formation.
Therefor, a technology has been investigated in which using a porous magnetic core particle having fine holes (hereinafter referred to also as pores) on the surface, a resin is filled in the fine holes and also physical properties such as porosity are specified to reduce the weight of a carrier particle (for example, refer to Patent Documents 3 and 4). Further, a carrier has been studied in which a resin is injected into holes and thereby adhesion properties between a porous magnetic core particle and the resin is enhanced to stably carry out large-volume printing at a level of hundreds of thousands of sheets (for example, refer to Patent Document 5).